1. Field of the Invention
The present invention relates to a process of preparing polycarbonates, more specifically to a process of preparing polycarbonates by solid state polymerization using microwave radiation, which comprises steps of preparing polycarbonate prepolymer having a certain range of viscosity average molecular weight; converting said polycarbonate prepolymer into crystalline particles having a certain degree of crystallinity; and producing polycarbonates by solid state polymerization of said crystalline particles by applying microwave radiation. In particular, the use of microwave radiation during solid state polymerization enables to maintain the internal temperature of a reactor uniformly by using the heat produced from the reactants themselves. Consequently, it also drastically reduces melt adhesion among polycarbonate prepolymer particles during solid state polymerization, thus resulting in the production of high quality polycarbonates.
2. Description of the Prior Art
Polycarbonates are known to have excellent properties in terms of transparency, impact resistance, mechanical strength and heat resistance, and thus have been widely used in industry in manufacturing transparent sheets, packaging materials, vehicle bumpers, compact discs and the like.
There are generally three different methods in preparing polycarbonates: interfacial polymerization method, melt polymerization method, and solid state polymerization method. In an interfacial polymerization method, polycarbonates are produced by vigorous mixing aqueous bisphenol A(BPA) solution substituted with sodium with a phosgene-containing organic solution. However, this method has not been shown very advantageous in that the phosgene used as a starting material is an extremely toxic gas. In addition, the solvent being used in the polymerization reaction is volatile chlorine containing organic solvent, so that it can pollute the air as well as the working environment. Furthermore, the polycarbonate obtained according to this method shall contain the salts produced during polymerization and remaining un-reacted reactants, which should be washed off with excessive amount of water. After washing, it should be dried again. However, it has been commercially widely used because it has an advantage to produce products having various characters and clear color.
A melt polymerization method, which produces polycarbonates by direct polymerization of starting materials in melt state under vacuum without using any solvents, has been introduced to solve the above problems. In this method, it is quite necessary to remove phenol, a reaction by-product of the polymerization reaction, in order to increase the molecular weight of polycarbonates being produced. However, the viscosity of reaction mixtures also drastically increases and the removal of phenol becomes more difficult as the melt polymerization goes on. Therefore, this method requires a powerful vacuum system sufficient to get rid of phenol from highly viscous reaction mixtures and a facility to stir the viscous reaction mixtures. And further, color of the product becomes deteriorated due to high reaction temperature. Solid state polymerization is a method designed to produce polycarbonates of high molecular weight having high purity and excellent color by polymerizing at reaction temperature below the melting point in solid states, comprising the formation of polycarbonate prepolymers having relatively low molecular weight, converting them into solid particles, and finally producing polycarbonates via solid polymerization (U.S. Pat. Nos. 5,266,659 and 5,717,056).
The solid state polymerization method performs a polymerization by supplying the heat required for the reaction from external sources by electric heater, a heat carrier, or a heating gas. However, these conventional solid state polymerization methods do not seem to deliver the desired heat uniformly enough into the reactor to maintain uniform reaction temperature distribution throughout reaction medium and thus result in either melt adhesion of polycarbonate prepolymer particles due to local overheating or insufficient polymerization rate due to locally reduced reaction temperature. These local melt adhesion and low temperature eventually make unable to obtain high quality polycarbonates.
Consequently, a new method of preparing polycarbonates is highly required in order to produce high quality polycarbonates with high molecular weight that can control the internal temperature of a reactor well and also increase the polymerization rate as compared to that of a conventional solid state polymerization methods of external heating.
To solve aforementioned problems of the conventional methods of preparing polycarbonates, the present invention was completed by taking preparation steps comprising a) preparing polycarbonate prepolymers having a viscosity average molecular weight of 4,000-18,000 g/mole through conventional interfacial polymerization of dihydroxyaryl compounds and phosgene or melt polymerization of dihydroxyaryl compounds and diarylcarbonates; b) converting said polycarbonate prepolymers into crystalline particles having 5-50% of crystallinity; and c) producing polycarbonates by solid state polymerization of said crystalline particles by applying microwave radiation for heat generation during said solid state polymerization reaction, thus resulting in production of high quality polycarbonates with high molecular weight and high purity within a short period of time.
Consequently, the object of this invention is to provide a method of preparing polycarbonates by solid state polymerization of polycarbonate prepolymers having desired viscosity average molecular weight and crystallinity using microwave radiation instead of external heat sources, which not only expedites the rate of solid state polymerization but also simplifies the process directed to the production of polycarbonates with high quality.
The present invention relates to preparing polycarbonates by solid state polymerization comprising the following steps:
a) preparing polycarbonate prepolymers having a viscosity average molecular weight of 4,000-18,000 g/mole of through conventional interfacial polymerization of dihydroxyaryl compounds and phosgene or melt polymerization of dihydroxyaryl compounds and diarylcarbonates;
b) converting said polycarbonate prepolymers into crystalline particles having 5-50% of crystallinity; and
c) producing polycarbonates by solid state polymerization of said crystalline particles by using microwave radiation.
The detailed description of the present invention is given hereunder.
The present invention relates to a process of preparing polycarbonates having high molecular weights by using microwave radiation wherein the solid state polymerization is conducted not by supplying heat from the external sources but by the heat generated from the inside of polycarbonate prepolymer particles, thus enabling to simplify the polymerization process, controlling the reaction temperature easy, reducing the melt adhesion of polycarbonate prepolymer particles, and increasing the polymerization rate.
The more-detailed description of the present method of preparing polycarbonates is given hereunder.
The first step is to prepare polycarbonate prepolymers having a viscosity average molecular weight of 4,000-18,000 g/mole by performing the interfacial polymerization method, wherein polycarbonate prepolymers are produced by vigorous mixing aqueous bisphenol A(BPA) solution substituted with sodium with a phosgene-containing organic solution or by the melt polymerization, wherein polycarbonate prepolymers are produced by melting dihydroxyaryl compounds and diarylcarbonates at 160-180xc2x0 C., reacting at an increased temperature of 200-280xc2x0 C. under atmospheric pressure, and further reacting at vacuum pressure below 1 torr.
The typical reaction conditions for preparing said polycarbonate prepolymers are same as those of conventional interfacial polymerization and melt polymerization for polycarbonate polymers. In conducting the melt polymerization for the preparation of polycarbonate prepolymers, the molar ratio of diarylcarbonates, expressed in the following formula (2), to dihydroxyaryl compounds, expressed in the following formula (1), is 1-1.3.
HOxe2x80x94Ar1xe2x80x94Zxe2x80x94Ar2xe2x80x94OHxe2x80x83xe2x80x83(1)
In the above formula (1), Ar1 and Ar2 represent the same or different phenyl group or its derivatives; and Z represents a single bond or xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94CON(R1)xe2x80x94 or xe2x80x94C(R2R3)xe2x80x94 linkage; R1, R2 and R3 represents H or xe2x80x94(CH2)nCH3 respectively; and n is an integer in the range of 0-4.
Ar3xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94Oxe2x80x94Ar4xe2x80x83xe2x80x83(2)
In the above formula (2), Ar3 and Ar4 represent the same or different phenyl groups or its derivatives.
In the melt polymerization, the molar ratio of diarylcarbonates to dihydroxyaryl compounds is related with the molar ratio of phenyl end groups to hydroxyl end groups, both of which are located at the end of polycarbonate prepolymers after melt polymerization. If the molar ratio of diarylcarbonate to dihydroxyaryl compounds is smaller than 1, it results in color deterioration of the resulting polycarbonate prepolymers due to the presence of excessive amount of unstable terminal hydroxyl groups in the resulting polycarbonate prepolymers. If it exceeds 1.3, however, it becomes difficult for the polycarbonate prepolymers to increase their molecular weights during solid state polymerization reaction due to the presence of excessive amount of terminal phenyl end groups in the resulting polycarbonate prepolymers.
In conducting the interfacial polymerization, the molar ratio of phosgene to diarylcarbonates of formula (2) is 1-1.5. The molar ratio of phenyl end groups to hydroxyl end groups in polycarbonate polymers can be controlled by using compounds as shown in chemical formula (3) having only one hydroxy group.
Rxe2x80x94Ar5xe2x80x94OHxe2x80x83xe2x80x83(3)
In the above formula (3), R represents H or an alkyl group having C1-C8; and Ar5 represents phenyl or its derivatives. Phenol or its derivatives can be used for this purpose.
Polycarbonate prepolymers for the solid state polymerization in the present invention are preferred to have viscosity average molecular weight 4,000-18,000 g/mole and the molar ratio of terminal phenyl/hydroxyl (xe2x80x94Ph/xe2x80x94OH) of range from 90/10 to 10/90, from 70/30 to 30/70 more preferably. If the viscosity average molecular weight of the prepared polycarbonate prepolymers is lower than 4,000 g/mole, the prepolymer particles become so minute that melt adhesion can easily occur during the solid state polymerization and also they may be easily sucked out by gas or vacuum. If the viscosity average molecular weight of the prepared polycarbonate prepolymers is greater than 18,000 g/mole, it becomes difficult to convert them into small crystalline particles.
In solid state polymerization, the terminal phenyl and hydroxyl groups of the polycarbonate prepolymers react with each other to increase the molecular weight of polycarbonate and generate phenol as a by-product. If the molar ratio of terminal phenyl/hydroxyl groups is off the range, the molecular weight of polycarbonate prepolymers does not increase well.
The second step is to impart crystallinity in a predetermined range to the above-prepared polycarbonate prepolymers. The methods for preparing crystalline particles are (i) precipitation of polycarbonate prepolymers by pouring molten polycarbonate into a non-solvent and (ii) treatment of crushed polycarbonate prepolymers particles having a size ranging from 10 xcexcm to 5 mm in a non-solvent with vigorous mixing. The size of the resulting crystalline prepolymer particles affects not only the magnitude of total surface area but also the distance that phenol, a by-product in solid state polymerization, is diffused to the surface. If the particle size is too small, a melt adhesion among prepolymer particles will occur during solid state polymerization. If the particle size is too big, on the other hand, the surface area is decreased and the phenol is not easy to diffuse out to the surface, thus resulting in the decrease of the rate of polymerization in the solid state polymerization, eventually not enabling to attain the increase in molecular weight of thus obtained polycarbonate prepolymers.
For easy handling and reduced adhesion, prepared crystalline polycarbonate particles can be agglomerated to make bigger porous pellets by pressing.
In performing solid state polymerization, polycarbonate prepolymers are converted into polymers in solid state and the solid state polymerization is generally performed at a temperature a little lower than the melting temperature (Tm).
However, because polycarbonate prepolymers are usually non-crystalline materials and have no definite melting point, they have fluidity and tend to fuse each other at the solid state polymerization temperature. To solve this matter, pretreatment to impart polycarbonate prepolymers have an appropriate degree of crystallinity before solid state polymerization is recommended. Therefore, it is very important for the polycarbonate prepolymers to have a proper degree of crystallinity and thus the inventors of the present invention used polycarbonate prepolymers having a crystallinity of 5-50%. If the crystallinity of polycarbonate prepolymers is below 5%, there would occur a melt adhesion of polycarbonate prepolymer particles during the solid state polymerization, thus making it difficult to remove phenol and eventually unabling to obtain high molecular weight polycarbonates as well as impeding continuation of the solid state polymerization. In contrast, if crystallinity of polycarbonate prepolymers is higher than 50%, the rate of solid state polymerization becomes slow due to a decrease in the amorphous parts of polycarbonate prepolymers wherein the desired polymerizations would occur.
There are several methods to impart crystallinity to polycarbonate prepolymers such as 1) a method to crush polycarbonate prepolymers into particles and crystallize by vigorous stirring them in a non-solvent, 2) a method to precipitate polycarbonate prepolymers by pouring molten polycarbonate prepolymer into a non-solvent directly, 3) a method to add polycarbonate solution into a non-solvent or lower the temperature of a polycarbonate prepolymer solution, and 4) a method to crystallize by add shear force to molten polycarbonate prepolymer.
The degree of crystallinity of polycarbonate prepolymer particles can be controlled by simultaneously adjusting one or more of the factors involved in the crystallization process; the kinds and amount of non-solvents, particle size, temperature of crystallization, and magnitude of shear force. Especially, when the crystallization is performed in a non-solvent environment in particular, crystalline polycarbonate prepolymer particles with relatively large surface area and many pores are obtained. This porosity is very beneficial in removing phenol, a by-product of the solid state polymerization.
The final step of the present invention is to prepare polycarbonates having high molecular weight by performing solid state polymerization by applying microwave radiation in the range of about 900 MHz to 2.45 GHz to the above crystalline prepolymer particles.
When microwave radiation is applied to the above crystalline polycarbonate prepolymer particles, the polar moiety of the crystalline polycarbonate prepolymer particles synchronizes with the microwave and generate heat thus increasing the overall temperature of polycarbonate prepolymer particles. The solid state polymerization is conducted for 1-72 hrs while maintaining the intensity of the microwave applied so that the internal temperature of solid state polymerization reactor can be maintained at 180-250xc2x0 C. The solid state polymerization can be generally performed at most temperature conditions without particular limitations as long as the solid state polymerization can be proceeded at a relatively acceptable rate and the crystalline prepolymer particles can be maintained in solid state.
However, if the reaction temperature goes above 250xc2x0 C., crystalline polycarbonate prepolymer particles would undergo a melt adhesion and it is difficult to obtain polycarbonate polymer of high molecular weight and high quality. If the reaction temperature goes below 180xc2x0 C., the solid state polymerization reaction rate becomes too poor. Therefore, it is important to maintain the solid state polymerization reaction temperature within a preferable range from 180xc2x0 C. to 250xc2x0 C.
As an optional way to remove phenol more effectively, such methods as inert gas sweeping through solid state polymerization reactor or vacuum application may be used while applying microwave radiation for heat generation. Especially, the phenol removal can be performed more effectively when inert gas sweeping is used while vacuum is maintained. The inert gas that can be used in the present invention can be a single gas or a mixture of gases selected from the group consisting of a nitrogen gas, a helium gas, and an argon gas. When vacuum is applied along with microwave radiation, it is preferred to maintain the vacuum of range from 3 torr to 700 torr, and from 10 torr to 300 torr more preferably.
However, if the vacuum goes below 3 torr, plasma flame occurs and the solid state polymerization cannot be performed well.
The polycarbonates prepared in the present invention have viscosity average molecular weights of 12,000-100,000 g/mole, which are much larger than those obtained by using the conventional solid state polymerization method within a short period of time.
Especially, the present invention could resolve the problems of conventional solid state polymerization such as melt adhesions and low polymerization rate by utilizing the heat generated from the inside of the polycarbonate prepolymer particles and removing phenol by optionally introducing either inert gas flow or vacuum during the solid state polymerization.